Tracking the Internet’s BGP Table

Geoff HustonTelstra

January 2001

Methodology The BGP table monitor uses a

router at the boundary of AS1221 which has a default-free eBGP routing table

1. Capture the output from “show ip bgp” every hour

2. Perform analysis of the data(and then discard the raw dump!)

3. Update reports at www.telstra.net/ops/bgp

The BGP Table

Phases of Growth

Exponential growth CIDR DeploymentCIDR-based Growth Post-CIDR

Exponential Growth

Growth Characteristics Short term route fluctuation is an

absolute value (not a % of total routes) of 1,000 – 2,000 routes

Routed Address Space

Large fluctuation is due to announcement / withdrawals of /8 prefixes12 months of data does not provide clear longer growth characteristic

980000000

1000000000

1020000000

1040000000

1060000000

1080000000

1100000000

1120000000

1140000000

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Routed Address Space (/8 Corrected)

Annual compound growth rate is 7% p.a.Most address consumption today appears to beocurring behind NATs/8 Corrected Data

Average size of a routing table entry

The BGP routing tale is growing at a faster rate than the rate of growth of announced address space

/18.1

/18.5

Number of AS’s in the table

Exponential growth is evident in a longer term view of the AS deployment rate

AS Number Usage Projection

0

10000

20000

30000

40000

50000

60000

Oct

-96

Feb

-97

Jun-

97

Oct

-97

Feb

-98

Jun-

98

Oct

-98

Feb

-99

Jun-

99

Oct

-99

Feb

-00

Jun-

00

Oct

-00

Mar

-01

Jul-0

1

Nov

-01

Mar

-02

Jul-0

2

Nov

-02

Mar

-03

Jul-0

3

Nov

-03

Mar

-04

Jul-0

4

Nov

-04

Mar

-05

Jul-0

5

Date

AS

Nu

mb

ers

in

us

e

Projection

AS Number Usage

AS Number Use - Extrapolation

Continued exponential growth implies AS number exhaustion in 2005

Number of distinct AS Paths

Observations for 99/00 Low growth in the number of routed addresses

0.6% growth / month (7% / year)

High growth in number of route advertisements3% growth / month (42% / year)

High growth in number of AS’s3.5% growth / month (51% / year)

Multi-homing on the rise? Track rate of CIDR “holes” – currently 40% of

all route advertisements are routing ‘holes”

This graph tracks the number of address prefix advertisements which are part of an advertised larger address prefix

Prefix Growth – Aug 00 to Oct 00

/16 6553 -> 6670 absolute growth = 117, relative = 1.79%/17 889 -> 936 absolute growth = 47, relative = 5.29%/18 1763 -> 1884 absolute growth = 121, relative = 6.86%/19 5704 -> 5984 absolute growth = 280, relative = 4.91%/20 3423 -> 3854 absolute growth = 431, relative = 12.59%/21 3621 -> 3856 absolute growth = 235, relative = 6.49%/22 5415 -> 5870 absolute growth = 455, relative = 8.40%/23 7298 -> 7788 absolute growth = 490, relative = 6.71%/24 49169 -> 52449 absolute growth = 3280, relative = 6.67%/25 208 -> 436 absolute growth = 228, relative = 109.62%/26 334 -> 606 absolute growth = 272, relative = 81.44%/27 469 -> 667 absolute growth = 198, relative = 42.22%/28 357 -> 452 absolute growth = 95, relative = 26.61%/29 579 -> 764 absolute growth = 185, relative = 31.95%/30 746 -> 1026 absolute growth = 280, relative = 37.53%

The largest significant relative growth in recent times is /20, tracking the allocation policy change in the RIRs

While the absolute number is low, the largest relative growth is in /25 prefixes, and /25 to /30 represent the greatest area of prefix growth in relative terms

Prefix Distribution

/24 is the predominant routing prefix

Prefix Growth Aug 00 – Jan 01

% growth Aug 00 – Jan 01

Conjectures….

BGP table size will continue to rise exponentially Multi-homing at the edge of the Internet is on

the increase The interconnectivity mesh is getting denser

The number of AS paths is increasing faster than the number of AS’s

Average AS path length remains constant AS number deployment growth will exhaust

64K AS number space in August 2005 if current growth trends continue

More conjecturing…. Inter-AS Traffic Engineering is being undertaken

through routing discrete prefixes along different paths -- globally (the routing mallet!)

RIR allocation policy (/19, /20) is driving one area of per-prefix length growth in the aggregated prefix area of the table

BUT - NAT is a very common deployment tool NAT, multihoming and Traffic Engineering is

driving even larger growth in the /24 prefix area

And while we are having such a good time conjecturing…

Over 12 months average prefix length in the table has shifted from /18.1 to /18.5

More noise (/25 and greater) in the table, but the absolute level of noise is low (so far)

Most routing table flux is in the /24 to /32 prefix space – as this space gets relatively larger so will total routing table flux levels

“Flux” here is used to describe the cumulative result of the withdrawals and announcements

This is fun – lets have even more conjectures… CIDR worked effectively for four years,

but its effective leverage to dampen route table growth and improve table stability has now finished

Provider-based service aggregation hierarchies as a model of Internet deployment structure is more theoretic than real these days

i.e. provider based route aggregation is leaking like a sieve!